The invention relates to a display device comprising an electroluminescent material between a first pattern of rowxe2x80x94or selectionxe2x80x94electrodes and a second pattern of columnxe2x80x94or dataxe2x80x94electrodes, at least one of the two patterns being transparent to the radiation to be emitted, the electrodes, together with the intermediate electroluminescent material, forming part of pixels at overlap locations of said electrodes, the device comprising a drive circuit which can supply a selected pixel with a substantially constant current during use.
Display devices of this type (matrices of organic LEDs, polymer LEDs) find an increasingly wider application in, for example, mobile telephones.
A problem in the drive of such matrices of organic LEDs is the capacitance associated with each LED, which capacitance is formed by the overlapping electrodes and the interpositioned layer(s) of organic material, as well as the capacitance of the drive leads. This is a problem because the LEDs are usually driven by means of current drive. A large part of the initial current which should actually flow through the relevant LED charges the capacitance associated with the LED so that the LED conveys too little current and consequently emits light initially at a too low luminance level. For larger matrices, the capacitance and the resistance of the drive leads also play a role and, due to long RC times, the desired setting level during a writing period cannot be achieved in some cases.
It is an object of the present invention to provide a solution to the above-mentioned problem.
To this end, a display device according to the invention is characterized in that a plurality of column electrodes is capacitively coupled to an electrode which is coupled to a voltage source via a switch. The voltage source is suitable for supplying a voltage jump.
By supplying at least one voltage jump in the forward direction of the LED during a selection period, the total capacitance associated with all LEDs in the relevant row can be rapidly charged in the desired direction so that the current through one (or more) LED(s) is very rapidly defined substantially exclusively by the associated current source(s). The voltage jump is preferably supplied directly at the start of the selection period.
With the aid of the same capacitive coupling, it is also possible to supply a voltage jump of opposite sign by the end of the selection period, so that the total capacitance associated with all LEDs in the relevant row is rapidly discharged or provided with such a charge that the LEDs which are no longer selected are reverse biased.
In this respect it is to be noted that U.S. Pat. No. 5,723,950 describes a similar principle for accelerating the adjustment of the LEDs in the forward direction. However, an extra circuit is provided for each column, which circuit comprises, inter alia, an operational amplifier with associated resistors and a capacitor. This leads to an unwanted number of extra components, even when such a precharge circuit is used for a group of two or more columns. Moreover, the drive transistor for each column must be able to supply the extra current determined by the precharge circuit; the transistors of the column driver must thus be designed for higher currents than those required for actual use. Since this usually requires extra space in the realization in an integrated circuit, these circuits become more expensive.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.